The effect of MnS inclusions on the crevice corrosion behavior of 18-8 stainless steel.

The relation between the composition of MnS inclusions and crevice corrosion behavior of 18-8 stainless steel has been investigated metallographically in chloride media by an electrochemical method. It was found that in highly corrosive environments the tendency to crevice corrosion depends on the amount of MnS, while in mildly corrosive environments it depends on the amount of MnS and the Mn/S ratio. The tendency to the cessation of crevice corrosion, however, depends on the amount of MnS and the Mn/S ratio, in both environments. The morphology of corrosion near the entrance of crevices in steels in which the Mn/S ratio is large was pitting caused by preferential dissolution of the MnS inclusions. In the inner parts of the crevices, however, the matrix is dissolved preferentially independent of the Mn/S ratio. It was also determined metallographically that the corrosion potential in the inner parts of crevices is less noble than that near the entrances.